Survey and zoning the concentration of heavy metals in the water of wells along the Sistan River from zero point border to Hamoun International Wetland

Introduction
The main activity of people in the Sistan region is agriculture and animal husbandry, which due to the drought and the unavailability of surface water and the intermittent flow of water in the Sistan River, the digging of irrigation wells has expanded in this area. The Sistan River continues of Afghanistan's Helmand River, which passes through agricultural and urban areas. Therefore, it is possible that some kinds of pollutants, especially heavy metals, can come from different sources. On the other hand, after entering the Sistan Plain, the river affects the water level of wells along the river. Therefore, in case of contamination, it may increase the concentration of heavy metals in wells. However, until now, the concentration of heavy metals in the water wells has not been investigated and only the salinity factor has been considered from the point of view of water quality. The quality of drinking water is associated with the concentration of physicochemical compounds such as nitrate, phosphate, various anions and cations, and organic and inorganic pollutants such as heavy metals. Drinking water contaminated with metals is turning into a primary health concern for human health care. Therefore, surveying the quality of water can be helpful in management. The aim of this study is to investigate the concentration of heavy metals in the wells along the Sistan River, in order to ensure the quality of water for agriculture, husbandry, and human consumption.
 
Materials and Methods
In this research, in order to ensure the appropriate quality of water in terms of the concentration of heavy metals and also the possible effect of EC and pH factors on their changes, the concentration of some elements and factors were mentioned in the water samples of 26 active wells. The samples were collected along the Sistan River (with a maximum distance of 1000 meters buffer zone) from the border point to the entrance to Hamoun Hirmand (Afzal-Abad branch, Lorg Bagh and Khwaje mountain) from January to March 2022. According to the investigation of the Afzal-Abad branch and the absence of wells, no samples were obtained from this section. At the same time as the sampling, the characteristics of the well, such as the year of establishment and the type of water usage were recorded. Sampling was done in three repetitions and during sampling pH, EC, TDS, and salinity variables were measured by a portable calibrated device and recorded three times. The collected samples were transferred to the laboratory in order to measure heavy metals in sterilized frosted glass containers. The standard method of the American Public Health Association (APHA) was used to measure each of the considered factors. After the preparation of samples, the concentration of heavy metals was measured with ICP. The obtained results were zoned using the normal kriging and co-kriging methods based on the selected model resulting from the prediction standard error. Geostatistical kriging methods (such as simple kriging, normal kriging, and co-kriging) were used for interpolation of heavy metals distribution. In the semi-variable analysis, the variability of the factors with respect to the spatial distance was organized by different linear, spherical, etc. functions using ArcGIS software.
 
Results and Discussion
The average amount of pH was measured at 8.31. The concentrations of salinity and TDS were 3.74 and 4.62 g l^-1, respectively. EC value was also measured as 6322 µS/cm. The average concentration of Cr, Fe, Ni, Cu, Zn, and Pb was also obtained at 2.64, 124, 10, 5.9, 33, 17, 1.43, and 2.79 µg l^-1 respectively. The trend of elements obtained Fe>Zn>Ni>Cu>Pb>Cr>Cd. The results indicated a low concentration of Cd, Ni, Cr, Cu, Fe, and Zn While Pb concentration was higher than the standard. The low concentration of the mentioned elements is due to the alkaline pH of water, which acts as a buffer and causes the elements to become insoluble and precipitate. Some dangerous elements such as mercury could not be measured due to their low concentration. The amount of salinity and EC factors also showed that the well water is not suitable for agriculture and livestock. The result of element zoning also showed that the concentration of metals increases from the border towards the lake. According to the age of the wells, it can be said that the reason for the decrease in the concentration of metals is due to the longer life of the wells and the chance of water mixing during the Sistan River water harvesting.
 
Conclusion
The result showed that the concentration of metals, except lead, is lower than the standard value. The low concentration of elements may be due to the alkaline condition of water, which acts as a buffer and causes the insoluble and finally precipitate. Or it can be due to the lower concentration in the bedrock, which can be obtained by surveying the geology of the bed of the wells. The variation in metal concentration among the sampling sites may be due to the age of the well. So up to the Sistan dam, where the wells are older, the concentration of elements is lower. It may be due to river flow, which can cause dilution of metals in wells during high water season. According to the amount of salinity and EC, irrigation with water can cause a quality decrease of soil that leads to loss of cultivated area. Moreover, the fact that water is not suitable for livestock and alternative sources should be introduced.